Materials such as glass, crystallized glass, and aluminum are employed in the substrates of information recording media such as the magnetic disks loaded in personal computers, portable digital audio equipment, and car navigation systems. Of these, glass-based materials such as glass and crystallized glass afford the advantages of good resistance to shock and better heat resistance than aluminum. They also afford adequate strength even when fashioned into thin sheets.
Methods of manufacturing information recording medium-use glass substrates include, for example, as described in Patent Reference 1, the method of melting a glass starting material by heating, feeding the glass melt obtained into a pressing mold, conducting pressing to mold a disklike material, and processing this material. There is also a method in which, instead of press molding, the glass melt is fashioned into sheets by a method known as floating and the sheetlike material obtained is then processed. Molding into sheets by floating is described, for example, in Patent Reference 2. A method of processing a sheetlike material is described in Patent Reference 3.
Methods of manufacturing crystallized glass substrates for information recording media, as described in Patent Reference 4, for example, include the method of press molding a glass melt, heat treating the material obtained to induce crystallization, and then processing it into a finished substrate.    [Patent Reference 1] Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-236831 or see English language family member U.S. Pat. No. 6,442,975    [Patent Reference 2] Japanese Examined Patent Publication (KOKOKU) Heisei No. 5-50446    [Patent Reference 3] Japanese Unexamined Patent Publication (KOKAI) No. 2006-99857    [Patent Reference 4] Japanese Unexamined Patent Publication (KOKAI) No. 2001-184624 or English language family member U.S. Pat. No. 6,426,311
Patent References 1-4 are expressly incorporated herein by reference in their entirety.
In recent years, the demand for thin information recording medium-use substrates has increased as the size of magnetic recording media has diminished. In the course of manufacturing substrates of constant thickness as set forth above, the production of materials without variation in sheet thickness is anticipated, since it suffices to render the thickness of the glass material uniform prior to processing.
The effect of variation in the sheet thickness increases as the glass substrate becomes thinner. Accordingly, as the substrate becomes increasingly thin, there is a need to reduce variation in the sheet thickness of the material to efficiently produce substrates with high precision from blank materials. Meeting this need requires thinly extending the glass to a constant thickness while it is in a molten state.
However, as molten glass is extended, the surface area per unit of volume increases and the thermal energy escaping through the surface increases sharply. Thus, the viscosity rises sharply, hampering extension. When molding a material from glass melt, in both the above-described pressing method and floating method, achieving a thin sheet requires the optimization of molding conditions such as the outflow temperature of the glass melt, the temperature distribution of the pressing mold, the pressure and timing of pressing, the temperature of the float bath, and the ambient temperature. These optimized conditions must also be maintained.
Even when these conditions are maintained constant, there are times when the sheet thickness of the material varies. As the sheets have become thinner, cases have arisen in which the variation in sheet thickness cannot be ignored. However, some of the reasons for which the sheet thickness of the material varies even when the above conditions are kept constant are unknown and have not been explored.
The present invention has for its objects to provide a method for manufacturing a disklike glass material for the mass production of information recording media substrate-use materials of constant sheet thickness, to provide a method for manufacturing an information recording medium-use substrate from the material thus manufactured, and to provide a method for manufacturing an information recording medium.